The 2 1/2-ton, $1.4 billion orbiter flew within 124 miles of Io's surface at 14:32 GMT, taking images and measurements of what is believed to be the most volcanically active world in the solar system.

"It looks like it went pretty well," said Jim Erickson, Galileo's project manager at Nasa's Jet Propulsion Laboratory. "We don't see any issues, and everything seems to have been done fine. This is one of the lucky ones."

Controllers had received much of the telemetry data within two hours of the flyby, and there were no indications that the radiation in the area of Jupiter's nearest moon affected the probe. It takes 42 minutes for a signal from Galileo to reach Earth.

Scientists are hoping to see how the moon has changed since two previous Io encounters in October and November, said Duane Bindschadler, Galileo's manager for science planning and operations at Nasa's Jet Propulsion Laboratory.

"We are looking at some areas we haven't seen at this high a resolution before," he said earlier. "The other thing we're doing is targeting the particularly active volcanoes, and hoping to see active lavas on the surface."

Using a myriad of instruments, scientists aimed to measure the temperature and potentially the composition of the lava on the fiery moon, he said. The readings and images will be returned over the next several weeks.

Galileo is now on its second extended mission since completing its primary goals in 1997. Fuel for manoeuvring is running low, navigation equipment is failing and the probe has encountered twice as much radiation as it was designed to withstand.

Mission officials, acknowledging that their plucky probe will not last forever, are incrementally increasing the danger of its encounters in an effort to squeeze it for as much scientific information as possible.

Galileo, which was launched in 1989, flew within 380 miles of Io in October, revealing more than 100 volcanoes, some of which spewed 2,700-degree-Fahrenheit (1,480-degree-Celsius) lava and vented gases miles into space.

A month later, Galileo flew within 186 miles of the surface. Its camera captured lava spurting more than a mile high. Engineers again were able to restart its computer after the radiation caused another shutdown of its science instruments.